1. Field of the Invention
The present invention generally relates to wireless communications networks and, more particularly, to indoor wireless networks in which battery discharge of mobile units is minimized by providing multiple receiving antennas and switching to a receiving antenna closest to the mobile unit.
2. Background Description
With the advent of in-building packet data wireless data communication standards, portable/personal computing devices such as laptop computers, PDAs (Personal Digital Assistants), palmtop computers, and the like will be used to access data through wireless communication links. In such systems, while wireless links eliminate cables and provide tetherless access, full mobility across the wireless network coverage area requires mobile devices to mostly operate on their batteries. It is generally desirable to maximize the interval between battery replacement or recharging by optimizing the discharge of batteries during different operations of the portable devices.
There are known methods for conserving battery charge for mobile devices such as laptop computers. For example, in laptop computers the display screen is shut off if no user input is sensed for a certain time duration, and also after certain period of inactivity a laptop computer goes to a low-power standby state.
With the advent of wireless radio connectivity provided by emerging standards, additional battery charge is consumed during communication over wireless links. While power required for receiving data is generally low, a considerable amount of battery power is used up for wireless transmissions from the mobile devices. As the required transmission power from a mobile increases with its distance from a receiving base unit (or access point), it is desirable that the distance between a mobile unit and the base station be as short as possible. However, in conventional pico-cells, as the base station antenna is located at a fixed point, the power to be transmitted by a mobile unit depends on its position within the cell. Therefore, there is a need for optimizing transmitted power for a mobile unit irrespective of its location within a cell.
Methods of reducing power consumption by mobile radio units by controlling transmitted power level are applied in practical cellular radio communication systems. Some newer methods for power control have also been proposed. For example, U.S. Pat. No. 5,710,981 describes a portable radio power control device and method using incrementally degraded received signals. The use of multiple antennas for improving performance of communication systems is well known. For instance, U.S. Pat. No. 5,884,192 describes a method of increasing sensitivity of a base station by providing a plurality of antennas and combining the received signals from the different antennas to form an estimate of the transmitted signal. Switching of antennas to optimize performance with reference to mobile units have also been proposed. U.S. Pat. No. 5,835,855 describes an antenna system with two antennas which are periodically switched to minimize throughput deterioration due to multipath effects. U.S. Pat. No. 5,628,052 describes an antenna switching method wherein the best antenna to be switched is determined by the mobile units. This requires a base station to transmit to a mobile unit via each of the different antennas and the mobile unit to inform the base station the best antenna based on parameters such as received signal strength and bit error rate. Generally methods such as this are aimed at improving throughput and error-free communications without regard to battery power consumption. However, in future short range cellular indoor wireless communication systems, a major concern is minimizing battery power consumption during wireless communications. In these systems, due to the use of techniques such as frequency hopping and very high frequencies (typically, greater than 1 GHz) generally throughputs required for the intended applications such as paging, electronic mail (E-mail), file transfers and browsing of simple Web pages are easily achieved.
It is therefore an object of the present invention to provide a method and apparatus which minimize battery discharge of mobile communication devices in short range wireless radio communication networks
According to the invention, there is provided a method and apparatus for minimizing battery discharge of mobile communication devices in short range wireless radio communication networks by switching receiving antennas to minimize transmitted power for a mobile communication device. The apparatus comprises multiple receiving antennas and at least one transmitting antenna, and power measuring and antenna switching circuitry. The receiving antennas are switched for each mobile communication device to minimize transmitted power required by a mobile communication device. Switching between receiving antennas is accomplished by measuring power of a signal received from a mobile communication device at each receiving antenna, and selecting a receiving antenna based on the measured power. A database of switchings is maintained for the mobile communication devices, and an initial selection of a receiving antenna is based on the last switching or a trend indicated by most recent switchings. Alternatively, the initial switching is to a centrally located receiving antenna.